Nucleoside analogues represent a major group of antitumour cytotoxic drugs. For example, the therapeutic use of pyrimidine nucleosides in the treatment of proliferative disorders has been well documented in the art. Commercially available antitumour agents of the pyrimidine series include 5-fluorouracil (Duschinsky, R., et al., J. Am. Chem. Soc., 79, 4559 (1957)), Tegafur (Hiller, S A., et al., Dokl. Akad. Nauk USSR, 176, 332 (1967)), UFT (Fujii, S., et al., Gann, 69, 763 (1978)), Carmofur (Hoshi, A., et al., Gann, 67, 725 (1976)), Doxyfluridine (Cook, A. F., et al., J. Med. Chem., 22, 1330 (1979)), Cytarabine (Evance, J. S., et al., Proc. Soc. Exp. Bio. Med., 106. 350 (1961)), Ancytabine (Hoshi, A., et al., Gann, 63, 353, (1972)) and Enocytabine (Aoshima, M., et al., Cancer Res., 36, 2726 (1976)). Cytarabine (ara-C) and fludarabine are the two most active drugs against leukemias, whereas, gemcitabine and 5-fluorouracil are active against a wide range of solid tumours.
The nucleoside analogues currently available for use in clinic are prodrugs which are not active by themselves. Upon entering cells, these nucleoside analogues are phosphorylated by nucleoside kinases and the phosphorylated metabolites are incorporated into DNA causing a pause in, or termination of, DNA synthesis. The close correlation between the degree of drug-induced cell death and the amount of incorporated analogue molecules in cellular DNA strongly suggests that the incorporation of these molecules into DNA is a key cytotoxic event (Azuma A et al; 2′-C-cyano-2-deoxy-β-D-arabino-pentafuranosyl cytosine: a novel anticancer nucleoside analog that causes both DNA strand breaks and G2 arrest; Molecular Pharmacology, 59 (4), 725-731, 2001).
The clinical effectiveness of nucleoside analogues appears to be influenced by multiple factors including the substrate specificities of nucleoside kinases, the expression levels of kinases in tumour tissues, and the rate of metabolic elimination by inactivating enzymes (Azuma A et al; ibid; Matsuda A and Sasak T, Antitumour activity of sugar-modified cytosine nucleosides; Cancer Science. 95 (2), 105-111, 2004). Rationally designed nucleoside analogues with improved biochemical properties may be more effective antitumour agents.
2′-C-Cyano-2′-deoxy-β-D-arabino-pentafuranosylcytosine (CNDAC) is a rationally designed analogue of deoxycytidine. It causes single-strand DNA breakage that cannot be repaired by ligation. This type of DNA damage is different from that caused by other nucleoside analogues such as ara-C and gemcitabine, which terminate or pause DNA synthesis at the site of incorporation [Azuma A et al; ibid]. This unique strand-breaking action seems to be the basis of CNDAC's ability to induce cell cycle arrest at the G2 phase, as distinct from the S-phase block seen with ara-C or gemcitabine. During the drug discovery phase, many derivatives of CNDAC were synthesized and investigated for stronger antitumour activity than CNDAC. For example, EP 536936 (Sankyo Company Limited) discloses various T-cyano-2′-deoxy-derivatives of 1-β-D-arabinofuranosylcytosine which have been shown to exhibit valuable anti-tumour activity. One particular compound disclosed in EP 536936 is 2′-cyano-2′-deoxy-N4-palmitoyl-1-β-D-arabinofuranosylcytosine (referred to hereinafter as “sapacitabine” or “CYC682” or “CS-682”); sapacitabine has been chosen for clinical development because of its broad range of antitumour activity in preclinical studies.
Sapacitabine, also known as 1-(2-C-cyano-2-deoxy-β-D-arabino-pentofuranosyl)-N4-palmitoyl cytosine, (Hanaoka, K., et al, Int. J. Cancer, 1999: 82:226-236; Donehower R, et al, Proc Am Soc Clin Oncol, 2000: abstract 764; Burch, P A, et al, Proc Am Soc Clin Oncol, 2001: abstract 364), is an orally administered novel 2′-deoxycytidine antimetabolite prodrug of CNDAC.

Sapacitabine has been the focus of a number of studies in view of its oral bioavailability and its improved activity over gemcitabine (the leading marketed nucleoside analogue) and 5-FU (a widely-used antimetabolite drug) based on preclinical data in solid tumours. Recently, investigators reported that sapacitabine exhibited strong anticancer activity in a model of colon cancer. In the same model, sapacitabine was found to be superior to either gemcitabine or 5-FU in terms of increasing survival and also preventing the spread of colon cancer metastases to the liver (Wu M, et al, Cancer Research, 2003: 63:2477-2482). To date, phase I data from patients with a variety of cancers suggest that sapacitabine is well tolerated in humans, with myelosuppression as the dose limiting toxicity.
Following oral administration, sapacitabine is converted to CNDAC by amidases and esterases in the gut, plasma, and liver. CNDAC can be converted to CNDAC-mono phosphate by deoxycytidine kinase which is thought to be the rate-limiting step in the formation of CNDAC-triphosphate (CNDACTP). CNDACTP is the active metabolite of sapacitabine and exerts its cytotoxic effects via the following mechanisms: a) potent inhibition of DNA polymerase, b) cessation of DNA strand elongation by incorporation into DNA strands, and c) breakage of DNA strands at the 3′-diester bond of CNDAC after its incorporation into the DNA. This latter mechanism is considered to be a novel effect that is not exhibited by other nucleoside analogues. CNDAC-phosphates can be degraded by cytidine deaminase and 5′-nucleotidase. However, compared with ara-C, CNDAC is a weak substrate of cytidine deaminase.
In addition to the antitumour activity of its metabolite, the parent drug sapacitabine itself is cytotoxic against a variety of cancer cell lines, including those lacking deoxycytidine kinase. This suggests that the antitumour activity of sapacitabine in vivo is likely to be mediated by both the parent drug as well as its active metabolite, CNDAC. The cellular pharmacology of sapacitabine is currently under investigation.
Sapacitabine and its active metabolite, CNDAC, showed a broad spectrum of activity against human tumour cells from various organs. In human tumour xenograft models, sapacitabine was active against a variety of tumours, and was especially effective against gastric, mammary, lung, colorectal, and hepatic tumour xenografts where tumour regressions were observed. Although sapacitabine showed a partial cross-resistance to ara-C-resistant tumour cell lines, it was active in vivo against P388 leukemia cell lines resistant to mitomycin C, vincristine, 5-FU, or cisplatin. In a mouse P388 leukemia model and in human xenografts of poorly differentiated gastric adenocarcinoma, sapacitabine exhibited much more potent antitumour activity than 5′-DFUR and gemcitabine.
Single-dose toxicity studies in rodents, and repeat dose studies of up to 3 months duration in mice and dogs have been completed. Sapacitabine has a direct toxic effect on rapidly proliferating cells, which is consistent with the known side effects of cytotoxic drugs. The major toxicities are hematopoietic, gastrointestinal, and testicular. The toxicities appear to be similar between single and repeat dosing, as well as between species.
In summary, sapacitabine, a rationally designed nucleoside analogue, may be a more efficacious antitumour agent than other nucleoside analogues. Its oral route of administration is more convenient for patients as compared with the intravenous administration route required by other nucleoside analogues.
The present invention seeks to provide new therapeutic applications for sapacitabine, and further seeks to provide improved dosing regimens for sapacitabine in the treatment of new and existing therapeutic applications.